The incidence of M. avium infection (MAI) in the SAIDS colony has increased from undetectable levels in 1988 to 18-25% in recent years. This opportunistic infection produces diarrhea, cachexia, and sometimes pneumonia in severely immunocompromized monkeys with remarkable similarity to MAI in children with AIDS. We detect M. avium by culture and PCR and characterize isolates from the environment (soil and water) and monkeys by molecular fingerprint analysis. To date, isolates from the environment rarely infect deep tissues of monkeys. In vitro testing of monkey macrophages demonstrates that cells co-infected with SIV and a common strain of pathogenic M. avium found in our monkey colony produces higher than expected levels of virus. Protein components of our pathogenic strain are being characterized.